1. Field of the Invention
The present invention relates to a micropump underpressure control device, particularly to a micropump underpressure control device maintaining a pressure difference between an inlet and an outlet of a micropump for better precision of the micropump and for preventing leaking out when the micropump is not in use.
2. Description of Related Art
Due to great progress of micromechanical and microelectronic production processes in recent years, along with research on a large scale, micropumps have been commercialized in inkjet printers, constituting the richest and greatest product and technique for research in fuel injection systems.
The main area of application of micropumps are printer heads. There are two types, heat bubble and piezoelectric micropumps. A heat bubble micropump has a flow path and an electric circuit on a silicon substrate and a plurality of ejection chambers. A surface thereof carries a nozzle plate. In a standby state, working liquid is held back in the ejection chambers due to capillarity. For operating the micropump, heat is generated by an electric current in the electric circuit, causing working liquid in ejection chambers to evaporate, forming bubbles which drive out liquid through ejection holes. A piezoelectric micropump has a substrate on which a membrane is laid, with chambers left in between. Piezoelectric material is laid on the membrane. Applying a varying electric voltage to the piezoelectric material displaces the membrane. Thus working liquid in the chambers is compressed and driven out through ejection holes in the substrate.
Currently, micropumps are mainly used in inkjet printers where precise dosing of ink is demanded. Therefore, precise control of the speed and quantity of dispensed ink is important.
Since a micropump furthers a tiny quantity of working liquid, pressure changes in the environment affect the precision thereof. A conventional micropump is usually installed in a casing, forming a closed device. Usage of spongy material results in underpressure against the exterior due to capillarity, preventing leaking of working liquid. After changing the environment, however, speed and quantity of dispensed ink are affected.
For precise operation of a micropump, it is required to maintain a fixed pressure difference between the inlet and the outlet thereof, so that speed and quantity of dispensed ink are stable and reproducible. A conventional micropump, however, does neither allow to control working pressure at the inlet nor external pressure at the outlet. Furthermore, after slowly releasing working liquid through the outlet, pressure in the casing (on the inlet side of the micropump) drops, increasing the difference to external pressure. Thus, with changing pressure difference between the inlet and the outlet, it is not possible to control speed and quantity of dispensed ink precisely.
It is the main object of the present invention to provide a micropump underpressure control device which is open to the exterior and automatically replenishes working liquid, while precisely controlling pressure difference between an inlet and an outlet, with inlet pressure being slightly lower than outlet pressure, resulting in continuing better precision of speed and quantity of dispensed ink, while during standby no working liquid leaks out.
For this, a control valve is mounted at an inlet tube of the micropump, being connected to a transmission tube. The transmission tube transmits pressure at an outlet to the control valve. The control valve has a membrane and a valve, the membrane following the pressure at the outlet, controlling the valve. Thus pressure of the working liquid at the inlet of the micropump is controlled.
By keeping pressure of the working liquid at the inlet of the micropump slightly below pressure at the outlet, the object of better precision of speed and quantity of dispensed ink is attained.